This invention relates generally to a large combined cycle gas turbine and reheat steam turbine power plant driving a load such as a large generator. More particularly, the invention concerns an improved method for preventing occurrence of destructive overspeed in such a combined cycle power plant upon sudden loss in load.
With large steam turbine generators the steam is normally reheated in a reheater section of the boiler after it passes through the high pressure turbine section and before it is expanded in the intermediate pressure and lower pressure turbine sections. Control valves at the inlet to the high pressure turbine are augmented by intercept valves between the reheater and the intermediate pressure turbine in order to intercept the high energy steam in the reheater before it can expand in the lower pressure turbines and contribute to overspeed. The control valves and intercept valves are backed up by main stop valves and reheat stop valves respectively as a second line of defense in the event of a control valve or intercept valve failure.
The reheater in the steam generator contains a significant mass of high energy steam which, in the absence of becoming trapped by the intercept valves flows into the lower pressure turbine sections and is converted to kinetic energy which contributes to overspeed. Some of this reheater steam contributes to overspeed by flowing through the intercept valves even while they are closing.
As the ratings of steam turbine generators have increased, the problem of limiting overspeed upon loss of load has become more severe. A variety of control devices have been developed either to close the valves faster or to begin closing them sooner than would occur by speed governor action alone. Closing of the valves when an unbalance first occurs between steam turbine torque and generator electrical load is disclosed in U.S. Pat. No. 3,198,954--Eggenberger et al., which is incorporated herein by reference. Other arrangements include application of additional electrical resistance as a load on the generator as suggested in U.S. Pat. No. 3,098,190--Spencer, et al. The foregoing patents are assigned to the present assignee. Various braking arrangements have also been suggested to add mechanical load; reversing turbine stages have been suggested.
Gas turbines do not have as difficult an overspeed control problem because a large portion of the total torque developed by the turbine is required to drive the air compressor. Furthermore, the torque required to drive the compressor is not a linear function of speed but increases approximately as the square of speed.
Combined cycle power plants having both a steam and a gas turbine have been proposed, with the steam and gas turbine connected in tandem on opposite ends of a generator. Such a combined cycle power plant is illustrated in U.S. Pat. No. 4,519,207--Okabe, et al., showing a two pressure level heat recovery steam generator of the non-reheat type supplying steam to a turbine connected in tandem with a gas turbine and driving a generator. Such a system would normally connect the rotors with "flexible couplings," which provide for thermal expansion and contraction. However, a flexible coupling may possibly fail in fatigue from any misalignment, which causes it to flex with each revolution. In the event of failure of the flexible coupling, the steam turbine could become completely separated from its load and therefore in a combined cycle plant, the steam turbine and gas turbine each have independent controls to prevent overspeed.
Japanese patent publication 62-38525 dated Aug. 18, 1987 discloses a non-reheat steam turbine/gas turbine combined cycle power plant having a single thrust bearing between the steam turbine and the gas turbine and coupled to a generator with rigid couplings. This publication concerns only arrangement of turbine units and discloses no steam generator or control system for the system.
Improved means are needed for controlling or reducing excessive overspeed in the event of loss of load in a large combined cycle gas turbine and reheat steam turbine power plant, without increasing the cost of the control system.
Accordingly, one object of the present invention is to provide an improved method for preventing overspeed in a large combined cycle gas turbine and reheat steam turbine power plant.
Another object of the invention is to provide an improved combined cycle power plant with a reheat steam turbine permitting elimination of the conventional reheat stop valve and intercept valve.